(1) Field of the Invention
The present invention relates to a device for measuring the thickness of slag on the surface of a liquid metal contained in a metallurgical vessel such as an ingot mould, notably of the continuous casting type.
(2) Description of the Related Art
Hereinafter in this description, the invention will be described with reference to the measuring of thickness of slag formed on the surface of a metal contained in an ingot mould without there being any limitation on the framework of the present invention whose design can be implemented in other metallurgical vessels containing a molten metal topped with a layer of slag, itself topped with a layer of non-molten powder. It will, however, be noted that the measurement of the level of slag in the ingot mould constitutes a preferred application of the present invention.
In the case of ingot mould casting, the conventional operation of continuous casting of a liquid metal schematically consists in continuously pouring the molten metal into a bottomless vertical tubular casting space of an ingot mould: this space is delimited by metal walls cooled by a circulation of water outside the casting space. The liquid metal flowing in the casting space is solidified at least on the surface, thus forming a skin, in contact with the walls of the ingot mould, before being continuously extracted from the ingot mould. The product extracted from the ingot mould is then core-cooled by spraying water and then cut to the desired length.
Conventionally, a covering powder is added to the surface of the molten metal bath. This melts on contact with the metal so that it forms two layers: a layer of molten powder of 3 to 15 mm in contact with the liquid metal, called slag, and a layer of non-molten powder. The purposes of the casting powders are mainly to thermally insulate the liquid metal bath, to prevent oxidation and to lubricate the ingot mould with the film of molten powder (slag) which flows along the walls of the ingot mould. Secondarily, it will be noted that the layer of non-molten powder thermally insulates the layer of slag and, consequently, very favourably influences the thermal homogeneity of the slag.
The exact composition of the powder depends on the parameters of the casting. This composition is an important parameter which will act on the balance between the melting of the slag and the consumption of powder. Controlling this balance involves measuring the slag thickness.
Traditionally, the thickness of the slag is measured manually by means of two metal rods. In the case where the liquid metal contained in the ingot mould is steel, it is known practice to use a rod made of steel and a rod made of copper of the same length. These rods are dipped vertically and partially into the liquid metal, to the same height. The rod made of steel melts on contact with the liquid steel, but not with the slag. The rod made of copper melts on contact with the slag. Thus, after the removal of the two rods, the comparison of their remaining length enables the measurement of the thickness of the slag. The thickness of the slag in fact corresponds to the length difference between the rod made of steel and the rod made of copper.
However, these measurements are done manually, which proves restrictive, because these operations mobilize an operator who cannot fully attend to another activity and have to be carried out regularly for the best possible monitoring. Furthermore, there is a certain risk of inaccuracy in the measurements performed, affecting the reproducibility of these measurements.